This invention relates to a method of manufacturing an input screen for an image intensifier which converts an image defined by radioactive rays such as gamma rays and X-rays into a visible pattern.
An image intensifier which converts high energy radioactive rays such as gamma rays and X-rays into a visible bright light pattern comprises an input screen and an output screen which converts an image of photoelectrons issued from the input screen into a visible image. This type of image intensifier is desired to produce a distinctly resolved visible image. To this end, it is required for an image of radioactive rays to be truthfully converted into an image of photoelectrons.
Generally, the input screen of the image intensifier comprises a substrate prepared from, for example, aluminum permeable to radioactive rays; an alkaline halide phosphor layer deposited on the substrate and enabled to effectively emit a light upon receipt of radioactive rays; and a photoemissive layer deposited on the phosphor layer and formed of, for example, an compound of antimony-cesium (Sb--Cs) sensitive to a phosphor light.
FIG. 1 shows a prior art input screen intended to truthfully convert an image of radioactive rays emitted from a source into an image of photoelectrons. The input screen of FIG. 1 comprises a substrate 11, phosphor layer 12 and photoemissive layer 15. A large number of cracks 13 are formed in the phosphor layer 12, which, therefore, actually consists of an aggregate of numerous phosphor blocks 14 defined by the cracks. This construction of the phosphor layer 12 causes beams 16 of a light to be scattered in such a direction as ensures their landing on the photoemissive layer 15, thereby elevating the resolution of the original image of radioactive rays. The light beams 16 are scattered only within the phosphor blocks 14, which act as a guide for the light beams 16 to be carried to the photoemissive layer 15.
One of the known processes of manufacturing the input screen is to deposit a phosphor layer 12 of cesium iodide on the aluminum substrate 11, impart thermal shocks to the phosphor layer 12, thereby producing cracks therein due to the different thermal expansion coefficients of the aluminum substrate 11 and phosphor layer 12.
However, an input screen manufactured by the abovementioned prior art process has the following drawbacks.
(1) Phosphor blocks defined by cracks have an unduly large size, presenting difficulties in being reduced, and consequently in elevating the resolution of the original image of radioactive rays. Further, it is difficult to ensure the reproducibility of the size of the phosphor blocks.
(2) It is difficult to produce cracks extending throughout the thickness of the phosphor layer 12 merely by stresses resulting from the different thermal expansion coefficients of the substrate 11 and phosphor layer 12. As a result, the phosphor layer 12 fails to fully display the action of guiding light beams to the photoemissive layer 15.
(3) For the reasons given under the above items (1), (2), an X-ray image intensifier using the prior art input screen has a degree of resolution of 28 to 30 line-pair/cm.
As mentioned above, the known input screen has the drawbacks that it is impossible to fully resolve the original image of radioactive rays; the size of phosphor blocks defined by cracks is difficult to reproduce, resulting in a decline in the property of guiding light beams to the photoemissive layer 15; and consequently the prior art input screen has been found unadapted for use with an image intensifier.